1. Field of the Invention
The present invention relates to a phase change memory (PCM) capable of solving a thermal disturbance issue.
2. Description of Related Art
A special problem of a conventional PCM is a thermal disturbance issue resulted with a reduction in node sizes. In short, in a PCM 100 illustrated in FIG. 1, when a pitch between any two adjacent bottom electrodes 102a˜c becomes smaller, a thermal conductance is usually generated as shown with dashed lines in FIG. 1. That is, originally, only a portion of a phase change (PC) material 106 contacted with the bottom electrode 102b under a top electrode 104 is changed phase. However, portions of the PC material 106 that are contacted with the bottom electrodes 102a and 102c are also changed phases due to the thermal conductance.
Phase change based memory materials have been widely used in read-write optical disks at present. These materials have at least two solid phases, including for example a generally amorphous solid phase and a generally crystalline solid phase. Laser pulses are used in read-write optical disks to switch between phases and to read the optical properties of the material after the phase change.
Phase change based memory materials, like chalcogenide based materials and similar materials, also can be caused to change phase by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher resistivity than the generally crystalline state, which can be readily sensed to indicate data. These properties have generated interest in using programmable resistive material to form nonvolatile memory circuits, which can be read and written with random access.
The change from the amorphous to the crystalline state is generally a lower current operation. The change from crystalline to amorphous, referred to as reset herein, is generally a higher current operation, which includes a short high current density pulse to melt or breakdown the crystalline structure, after which the phase change material cools quickly, quenching the phase change process, allowing at least a portion of the phase change structure to stabilize in the amorphous state. It is desirable to minimize the magnitude of the reset current used to cause transition of phase change material from crystalline state to amorphous state. The magnitude of the reset current needed for reset can be reduced by reducing the size of the phase change material element in the cell and of the contact area between electrodes and the phase change material, so that higher current densities are achieved with small absolute current values through the phase change material element.